1. Field of the Invention
The present invention relates generally to a glass sheet with a transparent conductive film in which a transparent conductive film is formed on a surface of a glass sheet. More particularly, the present invention relates generally to a glass sheet with a transparent conductive film having a low rate of change in sheet resistance before and after a heat treatment and to glass articles using the same.
2. Related Background Art
A glass sheet with a transparent conductive film formed thereon is used widely as a transparent conductor for a photovoltaic device such as a solar cell or the like and an image display device such as a liquid crystal display, a plasma display panel, or the like. For a building window, the glass sheet with a transparent conductive film is used as low-emissivity glass (Low-E glass), electromagnetic wave shielding glass, or the like. Similarly, for a vehicle window or a display-type refrigerator in shops, it is used as heat shading glass, glass with a defogger, or the like. As the transparent conductive film, a tin oxide film doped with a trace component such as fluorine, antimony, or the like, an indium tin oxide (ITO) film, or the like has been known. The transparent conductive film can be formed by a physical vapor deposition method such as a vacuum vapor deposition method, a sputtering method or the like, but it also can be formed by a spray method or a chemical vapor deposition (CVD) method accompanied by a thermal decomposition and oxidation reaction (a pyrolytic oxidation reaction).
Depositing a transparent conductive film on a glass sheet production line in a float process by the CVD method also has been known. For instance, JP 1-96044 A discloses a method of depositing a tin oxide film on a glass ribbon in a float bath. According to JP 1-96044 A, the tin oxide film deposited by this method has the lowest sheet resistance in a certain film thickness. JP 1-96044 A also describes the range of 200 to 350 nm as the thickness of the tin oxide film. In addition, JP 4-502305 A discloses the method of depositing a tin oxide film in a float bath and specifically describes a tin oxide film thickness of about 220 nm.
A transparent conductive film deposited by a pyrolytic oxidation reaction of a coating-film forming material, particularly a tin oxide film, is excellent in chemical resistance and can be formed with inexpensive raw materials. In general, however, the pyrolytic oxidation reaction makes it difficult to obtain a transparent conductive film with high conductivity (low specific resistance).
JP 63-184210 discloses a method for improving the conductivity of a transparent conductive film deposited by the pyrolytic oxidation reaction, through a heat treatment in a nitrogen atmosphere or a hydrogen atmosphere. JP 63-184210 also describes an example in which a sheet resistance of a tin oxide film with a thickness of about 200 nm is decreased by the heat treatment.
According to the method described in JP 63-184210, the sheet resistance of a transparent conductive film certainly is decreased once. In many cases, however, a glass sheet with a transparent conductive film formed thereon is reheated in a step to obtain a final product. For instance, a bending and/or tempering treatment of a glass sheet is conducted with the glass sheet being heated. In the manufacturing process of a multiple-glazing unit, a sealant requiring heating may be used in some cases. In a photovoltaic device or an image display device, a glass substrate with a transparent conductive film may be heated in a step of processing the substrate in some cases. Frequently, these heating processes are conducted in the air. When the substrate is heated in the air, the sheet resistance of the transparent conductive film such as a tin oxide film or the like tends to increase.
When the transparent conductive film is preheated in a nitrogen atmosphere or a hydrogen atmosphere, greater change in sheet resistance is caused after a heat treatment in the air. Consequently, the preheating instead provides uncertainty with respect to the conductivity of the transparent conductive film. Such uncertainty is not desirable for stabilizing characteristics of a final product.
The present invention is intended to provide a glass sheet with a transparent conductive film having characteristics that the change in conductivity due to heating in the air is suppressed while having high conductivity so that a final product is provided with stable characteristics.
In order to achieve the above-mentioned object, initially, the present inventors assume a heat treatment at 450xc2x0 C. in the air as a heat treatment required for processing the glass sheet. When a conventional glass sheet with a transparent conductive film is heat-treated for about three hours under this usual condition, the sheet resistance increases considerably. However, when consideration is given to the steps of processing the glass sheet with a transparent conductive film, it is desirable that the sheet resistance does not change greatly even after it is treated using such heating temperature and heating time as described above.
In order to secure the conductivity of the transparent conductive film, the present inventors set the thickness of the transparent conductive film to be at least 360 nm, preferably at least 400 nm instead of employing the preheating in a nitrogen atmosphere or a hydrogen atmosphere. The decrease in sheet resistance through the increase in thickness of the transparent conductive film tends not to be affected easily by the heat treatment in the air. Furthermore, the present inventors studied various aspects of the structure of an intermediate film formed between a transparent conductive film and a glass sheet, film formation conditions of the transparent conductive film, or the like. As a result, the present inventors succeeded in suppressing the rate of change in the sheet resistance of the transparent conductive film due to the heat treatment to a level of 15% or lower when taking the sheet resistance before the heat treatment as a reference.